This nonprovisional application claims priority under 35 U.S.C. § 119(a) on patent application No. 2003/208888 filed in Japan on Aug. 26, 2003, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an electrophotographic image forming device and a color misregistration correction method for the image forming device. More specifically, the present invention relates to (i) an image forming device that automatically adjusts phases of unevenness in rotation of image supporting bodies (the unevenness in rotation of the image forming bodies results in a color misregistration of a multicolor image when the multicolor image is formed by superimposing color component images formed on the image supporting bodies or on a transfer supporting body), and relates to (ii) a color correction method for the image forming device, the method being for automatically correcting the color misregistration of the multicolor image.
2. Description of the Related Art
Conventionally, image forming devices (e.g. digital color copying devices) form multicolor images by resolving inputted image data into color components, performing image processing on the color components, and superimposing images of the color components. With such image forming devices, the resultant multicolor images suffer from color misregistration if the images of the color components are not superimposed accurately. This often deteriorates image quality.
In conventionally known tandem-type image forming devices, an image forming section is provided for each color component, so as to form multicolor images more speedily. In the tandem-type image forming devices, image forming sections respectively form corresponding color component images. The corresponding color component images are then sequentially superimposed. In this way, multicolor images are formed. In such image forming devices, an image supporting body (photosensitive drum) in one image forming section differs from an image supporting body (photosensitive drum) in another image supporting body, in terms of rotational behavior. Therefore, the color component images are often transferred to different positions.
In the tandem-type image forming devices, a writing device is provided for each color, and each writing device forms an electrostatic latent image on an image supporting body provided for that color. By developing the electrostatic latent images, the color component images are formed. The color component images are then superimposed on a recording medium. Therefore, if the rotational axis (core) of the image supporting body is shifted, unevenness in rotation (a phenomenon that the surface velocity of the image supporting body is not constant) is caused. In such a case, color misregistration is likely to occur due to the unevenness in rotation. Thus, the color misregistration of multicolor images is a significant problem of the tandem-type image forming devices.
In view of this problem, some image forming devices perform adjustment for synchronizing rotational phases of image supporting bodies, so as to reduce the unevenness in rotation, and form excellent multicolor images with little color misregistration. The adjustment for synchronizing rotational phases is performed as follows. In each image forming station, an image for color misregistration correction is formed, and then outputted. An image formed by the outputs is checked visually. Based on the visual check, a correction value (value used for synchronizing the rotational phases of the image supporting bodies) that minimizes the color misregistration is calculated. Then, the correction value is inputted to an operating section. The adjustment is performed on the following occasions, for example: (i) before shipping the image forming devices after the image forming devices are manufactured, (ii) after parts of the image forming devices are manufactured and/or maintenance (e.g. replacement) of the parts of the image forming devices is performed, and/or (iii) before forming a multicolor image in the case where the image forming devices have not been used for a long time.
To prevent the color misregistration of multicolor images, some image forming devices detect the density of the pattern image after forming a pattern image, thereby controlling the rotation of the image supporting bodies. Other image forming devices control the timing of recording start signals. These image forming devices are disclosed in Patent Publications 1 to 3, for example.
In the image forming device disclosed in Patent Publication 1, an image pattern is formed by positioning a plurality of predetermined lines on each image supporting body at identical time intervals. Then, by using an optical sensor unit, the toner density of the image pattern is detected. Based on the result of detection, the unevenness in rotation in each of the plurality of image supporting bodies is detected. Based on the detected unevenness in rotation, the rotation of each of the plurality of image supporting bodies is controlled so as to synchronize the rotational phases of the plurality of image supporting bodies having the unevenness in rotation. In this way, the color misregistration is prevented.
In the image forming device disclosed in Patent Publication 2, the phase of each photosensitive drum is shifted in advance, so as to make it possible to shift the phase of driving unevenness. By shifting the phase, even if the distance between adjoining transfer positions corresponding to the image forming stations is set shorter than the circumference of the photosensitive drum, the variation of each photosensitive drum due to driving unevenness with respect to the printing medium passing through the transfer position can be congruent with the others. As a result, the color misregistration caused by the influence of the driving unevenness is prevented.
The image forming device disclosed in Patent Publication 3 performs color misregistration correction as follows. First, the density of an overlapping part of two pattern images (a pattern image formed on a photosensitive drum of the image forming section of a reference color component and transferred onto a transfer conveyer belt, and a pattern image formed and transferred by the image forming section of a color component to be adjusted) is measured. Then, an input of a recording start signal to a laser beam scanner for the color component to be adjusted is delayed or put forward, so that the measured value falls within an acceptable range around a density value at which the pattern images are superimposed at ideal accuracy.    (Patent Publication 1)     Japanese Publication for Unexamined Patent Application, Tokukai 2000-221749 (publication date: Aug. 11, 2000)    (Patent Publication 2)     Japanese Publication for Unexamined Patent Application, Tokukai 2000-137424 (publication date: May 16, 2000); U.S. Pat. No. 6,360,070    (Patent Publication 3)     Japanese Publication for Unexamined Patent Application, Tokukai 2000-81744 (publication date: Mar. 21, 2000); U.S. Pat. No. 6,148,168
However, the image forming device of Patent Publication 1 detects the unevenness in rotation of the image supporting body with respect to each image forming unit, and the rotational phase of each image supporting body is controlled in accordance with (i) a reference pattern provided for each color and (ii) detected information on the unevenness in rotation. Therefore, the phase of unevenness in rotation is obtained with respect to each image supporting body. As a result, there is a problem that a computing unit or the like device is required.
Moreover, even if there is unevenness in rotation, density does not vary significantly within a pattern image formed by a single image supporting body for the purpose of detecting the unevenness in rotation. Therefore, the detection is difficult in the case where the density variation within the pattern image formed by a single image supporting body is detected with respect to each image forming unit. In addition, because the pattern image needs to be formed in each image forming unit, there is a problem that it is necessary to form pattern images respectively at four places corresponding to four colors (C, M, Y, and K).
In the image forming device of Patent Publication 2, the phases of driving unevenness in the photosensitive drums are shifted in advance by about 60 degrees each. Therefore, the driving unevenness of the photosensitive drums is not detected by detecting the density variation within the pattern or the like formed with respect to each color component. Therefore, there is a problem that it is difficult to control the rotational phases at high accuracy.
Moreover, in the image forming device of Patent Publication 3, a pattern formed on an image supporting body that is a target of correction (correction-target image supporting body) is superimposed on an image formed on an image supporting body that is to be a reference point (reference image supporting body), and this is performed while changing the formation timing of the pattern on the correction-target supporting body. That is to say, phase differences are not taken into consideration. Therefore, if there are phase differences among rotational movements of the image supporting bodies, there are problems that desired color misregistration correction cannot be performed, and/or that a long adjustment time is required due to errors occurring during adjustment.